Impact of receiver inter-frequency bias residual uncertainty on dual-frequency GBAS integrity

Dual-Frequency Ground-Based Augmentation Systems (GBAS) can be affected by receiver Inter-Frequency Bias (IFB) when Ionosphere-Free (Ifree) smoothing is applied. In the framework of the proposed GBAS Approach Service Type F (GAST-F), the IFB in the Ifree smoothed pseudorange can be corrected. However, IFB residual uncertainty still exists, which may threaten the integrity of the system. This paper presents an improved algorithm for the airborne protection level considering the residual uncertainty of IFBs to protect the integrity of dual-frequency GBAS. The IFB residual uncertainty multiplied by a frequency factor is included in the Ifree protection level together with the uncertainty of other error sources. To verify the proposed protection level algorithm, we calculate the IFB residual uncertainties of ground reference receivers and user receiver based on BDS B1I and B3I dual-frequency observation data and carry out a test at the Dongying Airport GBAS station. The results show that the proposed Ifree protection level with IFB residual uncertainty is 1.48 times the current protection level on average. The probability of Misleading Information (MI) during the test is reduced from 3.2 × 10⁻⁴ to the required value. It is proven that the proposed protection level can significantly reduce the integrity risk brought by IFB residual uncertainty and protect the integrity of dual-frequency GBAS.